Step 1: Wash and pit the plums. Chop them roughly into 1-inch pieces.
Step 2: In a large bowl, combine the chopped plums with the granulated sugar. Gently muddle the plums using a muddler or a wooden spoon to help release their juices. Cover the bowl and refrigerate for at least 4 hours, or preferably overnight. The plums will release a lot of syrup.
Step 3: Strain the plum mixture through a fine-mesh sieve into a clean bowl, pressing firmly on the solids to extract as much liquid as possible. Discard the plum solids (or save them for another use, like a compote!).
Step 4: Add the apple cider vinegar to the strained plum syrup. Stir well to combine. Taste and adjust sweetness or tartness if desired. Transfer the plum shrub concentrate to a clean, airtight bottle or jar. It will keep in the refrigerator for up to 2-3 weeks.
Step 5: To assemble the mocktail, fill a glass with ice.
Step 6: Add 2-3 tablespoons of the plum shrub concentrate to the glass. Adjust to your preferred sweetness and intensity.
Step 7: Top with sparkling apple juice, filling the glass. Stir gently to combine.
Step 8: Garnish with a slice of fresh plum, a sprig of mint, or a lemon twist for an elegant touch. Serve immediately and enjoy!